The present invention generally relates to hydrocarbon fuel tank safety systems and, more particularly, to maintaining inert gas in the ullage space of a fuel tank containing a hydrocarbon fuel such as aviation jet fuel.
The combination of fuel vapor and air existing in fuel tank ullage space (empty space above the fuel) can represent a significant safety hazard. For example, the National Transportation Safety Board concluded that the probable cause of the loss in 1996 of TWA flight 800 with all passengers and crew was an explosion of the center wing fuel tank, resulting from ignition of the flammable fuel/air mixture in the tank. The fuel vapor suddenly reacted with great violence with the oxygen present in the ullage space. The FAA report concluded that this type of accident represents a serious threat and that fuel tank inerting should be used to prevent future events of this type.
One approach to fuel tank inerting is to introduce nitrogen gas into the ullage space of the fuel tank. Nitrogen gas does not support fuel vapor oxidation and represents the current best method for on-board generation of an inert gas. Nitrogen can be separated from air using either membrane pressure swing adsorption (PSA) or temperature swing adsorption (TSA). Although these nitrogen fuel tank inerting systems require the consumption of energy to function, they do provide an adequate approach to fuel tank inerting.
Carbon dioxide can also be used to provide fuel tank inerting. Carbon dioxide can easily be generated by reacting a small amount of fuel with oxygen in an air stream, and then removing water that is co-generated. U.S. Pat. No. 3,847,298 to Hamilton teaches a method for generating an inert gas using catalytic combustion of fuel to form carbon dioxide. The liquid jet fuel used in aviation, however, typically contains a substantial amount of sulfur-containing components, which can lead, for example, to problems such as corrosion of metal components in the inerting system.
Another system for carbon dioxide generation is disclosed by Y. Limaye, on the Internet at www.fire.tc.faa.gov/ppt/systems/20051102_FAA_OBIGGS_ Presentation_condensed.ppt, (Phyre Technologies, dated Nov. 2, 2005). The system disclosed by Phyre Technologies uses fuel vapor in the fuel tank ullage space to provide both the oxygen and fuel vapor required for carbon dioxide generation. The low boiling (vapor fraction) of the fuel may contain a lower sulfur concentration compared to the method of Hamilton. The lower sulfur concentration can reduce impact of sulfur dioxide and sulfur trioxide byproducts on the system components.
Jet fuel has a very low vapor pressure, however, at some of the conditions leading to low temperatures encountered in the fuel tank of aircraft. In particular the temperature in the fuel tank may be very low during descent after a long cruise at high altitude. The small amount of fuel in the vapor phase in the ullage space during such a time would provide a very low potential for carbon dioxide generation.
As can be seen, there is a need for fuel tank inerting for aircraft safety. There is a need for fuel tank inerting that consumes a minimal amount of energy yet provides reliable generation of inert gases and that mitigates impact of sulfur byproducts on the operation of the fuel tank inerting.